Since the invention of the integrated circuit, the semiconductor industry has experienced continual rapid growth due to continuous improvements in the integration density of various electronic components (i.e., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size, allowing for the integration of more components into a given area.
These integration improvements are essentially two-dimensional (2D) in nature, in that the volume occupied by the integrated components is essentially on the surface of the semiconductor wafer. Although dramatic improvements in lithography have resulted in considerable improvements in 2D integrated circuit formation, there are physical limits to the density that can be achieved in two dimensions. One of these limits is the minimum size needed to make these components. Also, when more devices are put into one chip, more complex designs are required.
Three-dimensional integrated circuits (3D IC) are therefore created to resolve the above-discussed limitations. In a conventional formation process of 3D IC, two wafers, each including an integrated circuit, are formed. The wafers are then bonded with the devices aligned. Deep vias are then formed to interconnect devices on the first and second wafers.
Recently, through-silicon-vias (TSVs), also referred to as through-wafer vias, are increasingly used as a way of implementing 3D IC. Generally, a bottom wafer is bonded to a top wafer. Both wafers include integrated circuits formed over substrates. The integrated circuits in the bottom wafer are connected to the integrated circuits in the wafer through interconnect structures. The integrated circuits in the wafers are further connected to external pads through through-silicon-vias. The stacked wafers can be subjected to a sawing process to provide a plurality of stacked die structures.